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JJBE-1344; No. of Pages 9 ARTICLE IN PRESS

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Medical Engineering & Physics xxx (2007) xxx–xxx

Micro-ﬁne ﬁnishing of a feldspar porcelain for dental prostheses Xiao-Fei Song a, Ling Yin b,∗, Yi-Gang Han a, Hui Wang c a School of Mechanical Engineering, Tianjin University, Tianjin 300072, China b Department of Engineering, Building 32, Australian National University, ACT 0200, Australia c Analysis & Measurement Center, Tianjin University, Tianjin 300072, China Received 20 June 2007; received in revised form 10 September 2007; accepted 16 October 2007

Abstract Intraoral adjustment of ceramic prostheses involving micro-finishing using diamond burs is a critical procedure in restorative dentistry because the durability of a restoration depends on the finishing process and quality. Force, energy and surface integrity in micro-fine finishing of a feldspar porcelain versus operational parameters were investigated using a 2-DOF (two-degrees-of-freedom) high-speed dental handpiece and a fine diamond bur of 20–30 ␮m grits. The tangential and normal forces were measured as being significantly small in the ranges 0.18–0.35 N and 0.22–0.59 N, respectively. High specific finishing energy of 110–2523 J/mm3 was observed in material removal, particularly when decreasing either the depth of cut or the feed rate. Scanning electron microscopy observations indicated that the surfaces generated were mainly due to ductile flow; however, microfractures also occurred in porcelain. Surface roughness was measured as 0.43–0.74 ␮min terms of arithmetic mean value (Ra), decreasing with the depth of cut, but insignificantly changing with the feed rate (ANOVA, P > 0.05). Recommendations for clinical practice are made on the basis of our testing results. © 2007 IPEM. Published by Elsevier Ltd. All rights reserved.

Keywords: Micro-ﬁne ﬁnishing; Feldspar porcelain; Surface morphology; Finishing force; Finishing energy

1. Introduction trophic fracture had always originated from surface and subsurface damage in ceramic prostheses [10], which results Feldspar porcelains have been attractive materials in in a reduction in strength and lifetime of the restorations restorative dentistry because of their approximations to the [2,8,18–20]. Furthermore, wear studies show that the sur- appearances and functions of human enamels [1–6].A face roughness of the ceramic prostheses greatly influence machinable feldspar porcelain is one of the ceramic materials enamel wear [21]. Therefore, the diminution of finishing- for dental CAD/CAM [7–9]. However, this porcelain is brit- induced damage for good surface quality becomes a major tle in nature and susceptible to machining-induced damage task in restorative dentistry. [2,3,10–12]. Although high-speed dental handpieces/burs have been Studies have shown that the feldspar porcelains suffer routinely used in dentistry for over 30 years [22,23], lit- from extensive chipping defects and microcracks in dental tle work has been reported on their performance in dental CAD/CAM processes, due to their high amount of glassy operations, particularly concerning the amount of damage phase [2,13,14]. Also, in intraoral dental finishing of the introduced in ceramic prostheses depending on the dental porcelain using dental handpieces and burs, extensive chip- burs [8]. There is a practical interest in the characterization ping damage and subsurface damage were introduced in of the performance of diamond burs in intraoral resurfacing the feldspar porcelain when using coarse grit diamond burs of dental bioceramics in restorative dentistry [8]. Some stud- [15–17]. Analyses of failed crowns have proven that catas- ies have found that fine grit diamond burs can be applied for improvement of surface roughness and reduction of sub- ∗ Corresponding author. Tel.: +61 2 6125 8536; fax: +61 2 6125 0506. surface damage [15,24,25]. Although these studies provided E-mail address: [email protected] (L. Yin). insights in grit size effect on dental restorations, they also

Please cite this article in press as: Song X-F, et al., Micro-ﬁne ﬁnishing of a feldspar porcelain for dental prostheses, Med Eng Phys (2007), doi:10.1016/j.medengphy.2007.10.005 JJBE-1344; No. of Pages 9 ARTICLE IN PRESS

2 X.-F. Song et al. / Medical Engineering & Physics xxx (2007) xxx–xxx ignored the dynamic and variable aspects in clinical processes because they were conducted at fixed loads under static conditions [15,24,25], which entirely differ from changeable dental operations. Recent studies on in vitro dental finishing using coarse burs have found that the forces and handpiece speeds depended on the operational parameters [17,26]. However, the influence of operational parameters on micro-fine finishing process and quality using fine burs remains unknown, even though it is critical to dental restorations. In this paper we describe the in vitro micro-fine finishing of a feldspar porcelain using a 2-DOF high-speed dental handpiece and fine diamond grits under a wide range of conditions. Finishing force, energy, speed, surface roughness, and surface morphology were investigated as a function of the relevant dental operational parameters. Implications concerning the intraoral adjustment using fine diamond grits in Fig. 1. Dental handpiece, bur, and specimen in finishing. dental practice are considered. 2.3. Characterization methodology

2. Experimental procedure Tangential and normal forces were measured using a dynamometer, a charge amplifier, and a data acquisition 2.1. Dental material system. The normal force is in the depth of cut direction and the tangential force is in the feed direction. The bur The specimens were feldspar porcelain blocks of speed was obtained from the frequencies corresponding to 15 mm × 12 mm × 5 mm in dimension, Vita Mark II (Vita the largest amplitudes of the tangential and normal force Zahnfabrik, Germany). Their microstructure comprises a data in the frequency domain using a fast Fourier transform glass matrix and approximately 30% irregular feldspar crys- [26]. tals of sanidine, nepheline and anothoclase [17] of 1–7 ␮min The specific finishing energy u is defined as the energy size [27]. Their mechanical properties are: Vickers hardness expended per unit volume of material removed and calculated H = 6.2 GPa, Young’s modulus E = 68 GPa, fracture tough- as [29]: 1/2 σ ness Kc = 0.9 MPa m , and strength = 100 MPa [28]. F v u = t s (1) avwb 2.2. Micro-fine finishing where Ft is the tangential force, vs is the bur speed, a is the depth of cut, v is the feed rate, and b is the specimen Micro-fine finishing was conducted on a 2-DOF w thickness. F v is the finishing power and av b is the volume computer-assisted apparatus. A detailed description of t s w of material removed per unit time. the apparatus was given in a previous study [26]. The arithmetic mean surface roughness (R ) was mea- This apparatus included a computer-controlled x–y table a sured using a stylus profilometer (Taylor Hobson, UK). (TKQ8163P/50*50, Zhonghuan, China), a high-speed den- Surface morphology was examined by scanning electron tal handpiece (PA-S, NSK, Japan), a piezoelectric force microscopy (SEM, XL-30, Philips, Holland). For force and dynamometer (9257 A, Kistler, Switzerland), a charge ampli- roughness testing, three separate measurements were made fier (5006, Kistler, Switzerland) and a data acquisition system under each finishing condition to obtain mean values and (LMS SCADAS III 305, LMS International, Belgium). A new diamond bur of diameter of ds = 1.3 mm and grits of 20–30 ␮m (SF114, ISO 158/013, Shofu, Japan) was used. Table 1 The bur against the specimen in micro-finishing is illustrated Micro-fine finishing conditions Parameter Value in Fig. 1. The bur, rotating with peripheral speed vs,was moved in the long direction of the 12 mm × 5 mm surface at Dental handpiece NSK PA-S, high speed air-turbine dental handpiece Dental bur SF114, ISO 158/013, fine diamond grits of 20–30 ␮m a feed rate vw and a depth of cut a. The handpiece was driven at air pressure 0.17 MPa at the rotational speed of 357.3 krpm Dental material Vita Mark II V5-12 A1C, fine-particle feldspar porcelain when unloaded. Water was delivered to the finishing area at Air pressure 0.17 MPa a constant flow rate of 30 ml/min. The finishing tests were Water flow rate 30 ml/min conducted at feed rates of 12–36 mm/min and depths of cut Depth of cut 2 ␮m, 5 ␮m, 10 ␮m, 15 ␮m, and 20 ␮m of 2–20 ␮m. The micro-fine finishing conditions are summa- Feed rate 12 mm/min, 18 mm/min, 24 mm/min, 30 mm/min, rized in Table 1. and 36 mm/min

X.-F. Song et al. / Medical Engineering & Physics xxx (2007) xxx–xxx 3 standard deviations. A two-way factorial analysis of vari- ance (ANOVA) at a 5% signiﬁcance level was applied for statistical analyses.

3. Results

3.1. Finishing forces and force ratios

The tangential force Ft as a function of feed rate for different depths of cut is summarized in Fig. 2(a). It shows that tangential forces were small and in the range 0.18–0.35 N. At the depths of cut of 2 ␮m and 10 ␮m they remained rela- tively steady with the feed rate; at the depth of cut of 20 ␮m they increased slightly with the feed rate. At the feed rate of 12 mm/min, the change in tangential forces was not signiﬁ- cant (P > 0.05) as a function of depth of cut. The tangential force Ft as a function of depth of cut for different feed rates is plotted in Fig. 2(b). At the feed rate of 12 mm/min, the tangential force increased signiﬁcantly when the depth of cut increased from 2 ␮mto15␮m, but sharply decreased when the depth of cut was 20 ␮m. At the feed rate of 24 mm/min the force increased by about 53% from 0.22 N

Fig. 3. (a) Normal force Fn vs. feed rate; (b) normal force Fn vs. depth of cut.

to 0.34 N with the depth of cut. At the feed rate of 36 mm/min the force increased by about 25% from 0.24 N to 0.30 N. At the depths of cut smaller than 10 ␮m, the influence of the feed rate on tangential force was not significant (P > 0.05). The normal force Fn as a function of feed rate for different depths of cut is given in Fig. 3(a). It indicates that normal forces were in the ranges 0.22–0.28 N and 0.33–0.37 N for the different feed rates at the depths of cut 2 ␮m and 10 ␮m, respectively. At the depth of cut of 20 ␮m, the forces were 0.41–0.59 N for the feed rates of 18–36 mm/min. The normal force Fn as a function of depth of cut for different feed rates is shown in Fig. 3(b). It shows that normal forces increased with an increase in depth of cut for all feed rates, except for an abrupt decrease at the feed rate of 12 mm/min when the depth of cut was 20 ␮m. The effect of the feed rate on the normal force was not significant (P > 0.05), especially at the depths of cut smaller than 10 ␮m. The force ratio Fn/Ft as a function of feed rate for different depths of cut is given in Fig. 4(a), scaling to 1.0–1.7 for the depths of cut of 2–20 ␮m. The effect of the feed rate on the force ratio was not significant (P > 0.05). The force ratio as a function of depth of cut for differ- Fig. 2. (a) Tangential force Ft vs. feed rate; (b) tangential force Ft vs. depth of cut. ent feed rates is shown in Fig. 4(b). It increased with the

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Fig. 5. (a) Bur rotational speed vs. feed rate; (b) bur rotational speed vs. Fig. 4. (a) Force ratio Fn/Ft vs. feed rate; (b) force ratio Fn/Ft vs. depth of depth of cut. cut.

At the lower feed rates, the energy values were higher than depth of cut, by 19%, 74%, and 30% for the feed rates of those at the higher feed rates at any depth of cut. Moreover, 12 mm/min, 24 mm/min, and 36 mm/min, respectively, in the the energy decreased at more rapid rates when the depths of range 1.0–1.7. cut increased from 2 ␮mto10␮m than from 10 ␮mto20␮m.

3.2. Bur rotational speed and speciﬁc ﬁnishing energy 3.3. Surface roughness and morphology

The rotational speed of the bur as a function of feed rate Arithmetic mean roughness Ra as a function of feed rate for different depths of cut is plotted in Fig. 5(a). The speeds at depths of cut of 2 ␮m and 20 ␮m are summarized in Fig. 7. decreased with the feed rate at any depth of cut. Bur rotational At the depth of cut of 2 ␮m, Ra values were smaller than speed as a function of depth of cut for different feed rates is those at the depth of cut of 20 ␮m, in the range 0.43–0.54 ␮m plotted in Fig. 5(b). The speeds decreased with the depth of and 0.55–0.74 ␮m for the depths of cut 2 ␮m and 20 ␮m, cut at any feed rate. respectively. The effect of the feed rate on Ra was insignificant The specific finishing energy as a function of feed rate (P > 0.05). for different depths of cut is given in Fig. 6(a). The energy SEM micrographs of the finished surfaces at the depths of decreased with the feed rate at any depth of cut. At the depth cut 2 ␮m and 20 ␮m for the complete variation of feed rates of of cut of 2 ␮m, the energy dropped remarkably by 62% from 12–36 mm/min are shown in Figs. 8 and 9, respectively. The 2523 J/mm3 to 959 J/mm3. It is also noticeable that specific surfaces generated under the selected finishing conditions energy values at the depth of cut of 2 ␮m were 3–4 and 5–8 are similar, with large smooth areas and plough striations by times larger than those at the depths of cut 10 ␮m and 20 ␮m, ductile cutting. Microfractures were also observed on the sur- respectively. faces. Fig. 8 shows an increasing tendency to microfracture The specific finishing energy as a function of depth of cut with increasing feed rate. At lower feed rates, typical plas- for different feed rates is plotted in Fig. 6(b). The energy tic flow, similar to those in ductile removal, was observed; significantly decreased with the depth of cut at any feed rate. small fragmentations due to microfracture also occurred on

X.-F. Song et al. / Medical Engineering & Physics xxx (2007) xxx–xxx 5

4. Discussion

In clinical restorations, ceramic prostheses are resurfaced by fast-rotating multiple diamond grits at a particular feed rate and depth of cut or infeed. The forces generated between the diamond bur and the ceramic prostheses are related to the resurfacing parameter as well as to the material properties [30]. The overall resurfacing force is composed of forces acting on individual diamond grits as they indent and scratch the ceramic prosthesis to remove material from the surface by a combination of chip-formation, fatigue and fracture processes. The details of the removal process are determined by the nature of the abrasive particles (shape, strength, size, etc.), the properties of the ceramic prostheses (hardness, elastic modulus, fracture toughness, etc.), the magnitude of the interaction forces between the abrasive grits and the ceramic prosthesis, the time-dependence of the forces, the nature of surrounding environment (coolant and chips). In particular, force-related physics involving in dental resurfacing becomes more significant. Tangential and normal forces showed upward trends relative to depth of cut. This is consistent with common findings in conventional machining [31]. However, similar upward trends relative to the feed rate were not clearly evident. The force ratio Fn/Ft is associated with the coefficient of friction, and varied in the range 1–1.25 in the dental removal of enamel [32]. The obtained force ratios of 1–1.7 for porcelain, were Fig. 6. (a) Specific energy u vs. feed rate; (b) specific energy u vs. depth of close to those for enamel. But they were much lower than cut. those for the same porcelain when finishing with coarse diamond grits, where the force ratio values were 3–5 [17]. This indicates that more friction occurred in dental finishing pro- the flanks of the plastic flow, as shown in Fig. 8(a and b). At cesses using fine diamond grits than in finishing using coarse higher feed rates, more microfracture areas were visible, as grits. This may be attributed to more ductile flow involved in shown in Fig. 8(c–e). In comparison with Fig. 8, Fig. 9 shows the removal process, which resulted in more friction between that there is more microfracture occurring at the depth of cut the diamond grits and the porcelain material. 20 ␮m, especially at higher feed rates where more chipping Specific energy is a direct consequence of the removal damage and cleavage fracture were observed, as shown in mechanisms [33]. At smaller depths of cut or feed rates, cor- Fig. 9(d and e). responding to lower removal rates and smaller grit depths of cut, the specific energy was large, as shown in Fig. 6. Since specific energy is mainly expended by ductile flow (plowing) [34,35], the steep increase of specific energy at very small depths of cut can be attributed to an increased tendency for ductile flow. Surface roughness decreases when the ductile cutting mechanism prevails in material removal [36]. Therefore, a better surface finish might correspond to a larger specific energy due to more energy being expended by plastic deformation. The results of surface roughness also show this consistent trend in the current study. The smaller depth of cut, corresponding to larger specific energy, resulted in a better surface finish, as shown in Fig. 7. However, these specific energies in dental micro-fine finishing are significantly higher than those in conventional machining of ceramics or glasses where the specific energies normally ranged from 10 J/mm3 to 800 J/mm3 [35,37]. In spite of the better surface finish generated at the low depth of cut, high specific energies are Fig. 7. Arithmetic mean roughness Ra vs. feed rate. partially released in the form of heat [29,32]. Thus, the high

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Fig. 8. SEM micrographs of the finished surfaces at the depth of cut of 2 ␮m for feed rates of (a) 12 mm/min, (b) 18 mm/min, (c) 24 mm/min, (d) 30 mm/min, and (e) 36 mm/min. magnitude of specific energy can also produce high surface the material properties as follows [39]: temperatures, which sometimes may lead to surface damage [38]. This is particularly critical to dental restorations where E K 2 d = . c the surrounding tooth tissues are very sensitive and vulnera- c 0 15 H H (2) ble to high temperatures in intraoral environments with limits to coolant access. where E is the Young’s modulus, H is the hardness, and K is To simplify the model of material resurfacing, the physics c the fracture toughness. According to Eq. (2), the critical grit of the single diamond girt penetrating into the surface of the depth of cut for this porcelain is calculated to be 0.24 ␮m. material and scratching or abrasion is highly considered. As In abrasive removal, the grit depth of cut is defined as the penetration occurs into a brittle material, elastic displacement maximum undeformed chip thickness, which can be calcu- is followed by plastic flow and fracture. A central factor in lated as [29]: determining the finishing mode (ductile regime versus brittle) is the grit depth of penetration or cut. Brittle materials can 1/2 v 1/2 a 1/4 be removed in a ductile mode when the grit depths of cut are 3 w hmax = (3) below a critical grit depth of cut, dc, which is associated with C tan θ vs ds

X.-F. Song et al. / Medical Engineering & Physics xxx (2007) xxx–xxx 7

Fig. 9. SEM micrographs of the finished surfaces at the depth of cut of 20 ␮m for feed rates of (a) 12 mm/min, (b) 18 mm/min, (c) 24 mm/min, (d) 30 mm/min, and (e) 36 mm/min. where C is the number of active cutting points per unit area, θ grain dislodgement [30]. In the current study, it is very is the semi-included angle for the undeformed chip, which is difficult to observe the grain dislodgement and transgran- ◦ taken as 60 [29], vw is the bur feed rate, vs is the bur periph- ular fracture because the grain boundaries of the feldspar eral speed, a is the depth of cut, and ds is the bur diameter. grains were not easily visible due to their fusion with glass In material removal, better surface roughness results from matrix. Plastic flow in finishing of ceramics is similar with the smaller grit depths of cut [29]. According to Eq. (3), the grit machined plastic metal materials, and it looks smooth [30]. depth of cut is related to the fourth root of the depth of cut and For the diamond bur applied in the current study, the dia- to the square root of the feed rate. Therefore, surface rough- mond grits per unit area are estimated to be 2500 grits/mm2 ness is more influenced by the depth of cut than by the feed based on the SEM observations of the bur topography in rate. This explains why in the current study the influence of Fig. 10. Given that the fraction of active grits involving the depth of cut on surface roughness was significant, while removal is generally to be 1–10% [40], we assumed that the the effect of the feed rate on roughness was insignificant, as fraction of active grits for the fine bur is 5%. Therefore, C is shown in Fig. 7. approximately 125 grits/mm2. According to Eq. (3), the grit In removal of most polycrystalline ceramics, microfrac- depths of cut are estimated to be 0.068–0.22 ␮m, which were ture occurs via intergranular or transgranular fracture, or smaller than the critical grit penetration depth of 0.24 ␮m.

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induced heat. The micro-fine finishing mechanism for the feldspar porcelain was dominated by the ductile material removal mode, but a small degree of microfracture occurred. The finishing-induced microcracks on the surfaces are likely to propagate at high loads or stress concentrations. This suggests that the lifetime of porcelain prostheses can be improved by removing the microfractured layer with final polishing.

Acknowledgments

This work was supported by NSFC Project Grant No. 50475115. We thank Dr. Ulf Griesmann of NIST and Dr. Anthony Flynn of ANU for valuable comments.

Fig. 10. SEM topography of the used ﬁne diamond bur.

Conflict of interest This suggests that the material finishing of the porcelain was dominated by the ductile mode. However, it should be noted The authors have not entered into a commercial relation- that both Eqs. (2) and (3) were based on many assumptions. ship with any other individuals or organizations, that might For example, Eq. (2) assumes that the materials are isotropic include employment, consultancies, stock ownership, hono- and it therefore applies better to glasses than ceramics with raria, paid expert testimony, patent applications/registrations, grain boundaries [41]. Eq. (3) assumed a simple chip geom- and grants or other funding, other than that funding by the etry [29]. Furthermore, the diamond grits can function as National Natural Science Foundation of China Project Grant both sharp and blunt indenters in finishing. Upon contact, No. 50475115. a sharp indenter induces very high stress, which remains approximately constant regardless of depth. A rounded indenter induces low stresses on initial contact, which increases References with increasing penetration depth [42]. The occurrence of the microfractures in porcelain might be attributed to high [1] Kelly JR. Ceramics in restorative and prosthetic dentistry. Annu Rev pressure-induced stresses beneath the finished surfaces. Fur- Mater Sci 1997;27:443–68. thermore, certain planes in the feldspar crystals in porcelain [2] Thompson VP,Rekow DE. Dental ceramics and the molar crown testing have lower cleavage stresses than others. Thus, finishing near ground. J Appl Oral Sci 2004;12(sp.):26–36. [3] van Noort R. Dental ceramics. In: van Noort R, editor. Introduction to the fracture threshold can result in some areas that have a dental materials. 2nd ed. Oxford: Elsevier; 2002. p. 231–46. ductile response and some that have a component of brittle [4] Reiss B, Walther W. Clinical long-term results and 10-year response. This explains why a certain degree of microfrac- Kaplan–Meier analysis of CEREC restorations. Int J Comput Dent ture was observed on the porcelain surfaces finished in the 2000;3:9–23. ductile region using the fine grit diamond bur, as shown in [5] Bindl A, Mormann WH. An up to 5-year clinical evaluation of posterior in-ceram CAD/CAM core crowns. Int J Prosthodont 2002;15:451–6. Figs. 8 and 9. [6] Otto T, De Nisco S. Computer-aided direct ceramic restorations: a 10- year prospective clinical study of Cerec CAD/CAM inlays and onlays. Int J Prosthodont 2002;15:122–8. 5. Conclusions [7] Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652–61. [8] Thompson Rekow D. Engineering long-term clinical success of We investigated the fundamental features in micro-fine advanced ceramic prostheses. J Mater Sci Mater Med 2007;18:47–56. finishing of a feldspar porcelain for a typical range of den- [9] Tinschert J, Zwez D, Marx R, Anusavice KJ. Structural reliability of tal operational parameters. Higher feed rate and depth of alumina-, feldspar-, leucite-, mica- and zirconia-based ceramics. J Dent cut resulted in a more efficient finishing since the energy 2000;28:529–35. expended was less. However, higher feed rate and depth of cut [10] Harvery CK, Kelly JR. Contact damage as failure mode during in vitro testing. J Prosthodont 1996;5:95–100. likely result in more surface and subsurface damage. Better [11] Lawn BR, Deng Y, Miranda P, Pajares A, Chai H, Kim DK. Overview: surface finish and morphology were obtained at low depths of damage in brittle layer structures from concentrated loads. J Mater Res cut or feed rate, resulting in low finishing forces and high spe- 2002;17:3019–36. cific finishing energies. The high specific finishing energies [12] Lawn BR, Deng Y, Lloyd IK, Janal MN, Rekow ED, Thompson VP. may cause thermal damage in the surrounding tooth tissues, Materials design of ceramic-based layer structures for crowns. J Dent Res 2002;81:433–8. which can be a problem when the finishing is conducted [13] Sindel J, Petschelt A, Grellner F, Dierken C, Greil P. Evaluation of intraorally. Caution should be taken in micro-fine finishing subsurface damage in CAD/CAM machined dental ceramics. J Mater using fine grit diamond burs to avoid intense finishing- Sci Mater Med 1998;9:291–5.

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Please cite this article in press as: Song X-F, et al., Micro-ﬁne ﬁnishing of a feldspar porcelain for dental prostheses, Med Eng Phys (2007), doi:10.1016/j.medengphy.2007.10.005